Photodetector in Germanium on Silicon

ABSTRACT

A photodetector structure includes a silicon-based waveguide in which optical signals to be detected travel in a given direction and are confined therein and a germanium layer disposed in contact with a portion of the silicon-based waveguide so that an evanescent tail of the propagating optical signal in the waveguide is coupled into the germanium layer. In addition, the germanium layer includes a mesa having a length along the signal propagating direction and a width in a direction substantially perpendicular to the propagating direction, in which the width of said mesa is smaller than its length. The photodetector also comprises a first and a second metal contacts, the first metallic contact being located on the germanium layer, the said second metallic contact being located on the silicon-based waveguide, the first and second contacts being used to collect electrons generated by light absorption to obtain an output electric signal.

TECHNICAL FIELD

The present invention relates to a photodetector realized in germaniumon silicon, particularly a photodetector for near-infrared application.The selected geometries of the photodetector allow to achieve a goodresponsivity and excellent speed of the overall device.

TECHNOLOGICAL BACKGROUND

The introduction of fiber-based optical communication has brought agreat increase of long haul telecommunications: the inherent low cost,wide bandwidth and small attenuation of fibers are key factors inprevailing over copper wire. In short haul access network, however,fiber qualities are superseded by the current high costs of opticaltransceivers. These components are usually hybrid assembly of III-Vdevices such as lasers, modulators, photodiodes and waveguide.

In order to succeed, novel approaches for transceiver fabrication arerequired. Among others, silicon-based opto-electronics is attractivebecause of its potential low cost, scalability and reliability andintegration with the mature and unsurpassed silicon VLSI technology.

In addition, light detection in the near-infrared (NIR) region is ofextreme importance in optical telecommunications, particularly when highbit rates and low power levels are involved. It becomes thereforecrucial to employ NIR detectors that not only exhibit good sensitivityand speed in the spectral range on interest, but that can be closelyinterconnected to the to the driving/biasing and amplifying electroniccircuits. Since the most common platform for electronic processing ofsignals is based on silicon, the integration of NIR photodiodes onstandard silicon platform has been pursued in the past two-decades as aviable low-cost and high-efficiency solution to the growing request forcompact semiconductors microsystems for optical signal processing.

Several approaches have been proposed, such as hybrid integration ofIII-V based devices (silicon optical bench SIOB) or monolithicintegration of InGaAs on silicon.

Silicon-germanium (SiGe) has been considered a promising alternative toInGaAs, due to its large absorption coefficient in the NIR and goodcarrier transport properties. However, due to its relative latticemismatch, the epitaxy of SiGe requires the use of appropriate bufferlayers or other techniques which possibly hinder a seamless integrationwith CMOS Si-electronics. Nevertheless, a number of successful attemptshave been reported to date.

In “Si-based Receivers for Optical Data Link”, written by B. Jalali andpublished in Journal of Lightwave Technology, vol. 12, no 6, of June1994, pages 930-935, a Ge_(x)Si_(1-x) waveguide pin detectors grown byrapid thermal chemical vapour deposition is presented. A typical devicestructure consists of a Ge_(x)Si_(1-x)/Si multiple quantum wellabsorption layer, Si cladding layers approximately 1 μm thick, and n⁺and p⁺ contact layers.

One of the most appealing attempts of designing NIR photodetectors thatcan be integrated with standard semiconductor technology is based onpolycrystalline Ge mainly because of the low thermal budget required inthe device fabrication. Polycrystalline films are deposited at lowtemperatures which guarantee a good compatibility with standard Siprocessing. The deposited films exhibit absorption spectra similar tothose of monocrystalline Ge, but mobility and lifetimes are reduced.

In “Monolithic integration of near-infrared Ge photodetectors with Sicomplementary metal-oxide-semiconductor readout electronics” written byG. Masini et al. and published in Applied Physics Letters, volume 80, no18, of May 2002, pages 3268-3270, a monolithic integration of an arrayof near-infrared Ge photodiodes on Si complementarymetal-oxide-semiconductor (CMOS) electronics is reported. The chip,realized with standard very large scale integrated silicon techniques,hosts eight CMOS switches used to select one out of eight pixel. Thephotocurrent generated by the selected photosensitive pixel is fed to atransimpedance amplifier with an external feedback resistor R. Theoutput is a voltage corresponding to the light intensity at theinterrogated detector in the linear array. The latter encompasses eightpoly-Ge/Si heterojunction photodiodes, each of them realized between theevaporated poly-Ge and the n-well left exposed in the Si substrate. Siareas are connected to metal pads. Additionally, silver was evaporatedand lithographically defined in the shape of thin fingers on the centralportion of the poly-Ge, to lower the series resistance.

In “2.5 Gbit/s polycrystalline germanium-on-silicon photodetectoroperating from 1.3 to 1.55 μm” written by G. Masini et al. and publishedin Applied Physics Letters, volume 82, no 15, of April 2003, a fastpolycrystalline germanium-on-silicon heterojunction photodetector forthe near-infrared is described. The poly-Ge-on-Si photodiodes arefabricated by thermal evaporation of germanium on a silicon n-typesubstrate held at a temperature of 300° C. The thickness of theresulting Ge film is 120 nm. After deposition, the detector area isdefined by wet etching of square mesas 200×200 μm² in size. Thedeposition and lithographic definition of Ag contacts complete thefabrication. Light is coupled to the active detector area through thesubstrate, transparent at the wavelength of interest. Therefore thisdevice is fabricated for normal incidence detection and it exhibits a1.3 μm responsivity of 16 mA/W, dark current below 2 mA/cm² and anoperational speed higher than 2.5 Gbit/s.

Applicants attribute the low responsivity of this device to the smallactive region (i.e., depletion layer plus one diffusion length), whichcan be typically of 50 nm in Ge on Si, associated to the largeacceptor-like defect-density typical of polycrystalline germanium.Applicants believe that an increase in responsivity is expected inwaveguide geometry where the absorption efficiency depends on thedetector length rather than on the thickness of the active layer (suchas in normal incidence detectors).

In “Near-infrared waveguide photodetector based on polycrystalline Ge onsilicon-on-insulator substrates”, written by G. Masini et al., publishedin Optical Materials 17 (2001), pages 243-246, an integration of apoly-Ge photodetector with a waveguiding structure is depicted. Thisapproach allows the distributed absorption of the incoming light in thethin sensitive layer of the poly-Ge/Si heterojunction, thus increasingthe effective absorption length and efficiency. Bond and Etch-backSilicon-on-Insulator substrates with 2 μm thick n-type silicon and 1.5μm thick SiO₂ insulator are chosen as substrates. Polycrystalline Gefilms were grown by thermal evaporation using a 99.999% puritycommercial source. Film thickness is selected to be 120 nm. The deviceresponsivity was measured at normal incidence (shining light from thesubstrate) and in the waveguide configuration. In both cases asemiconductor laser emitting 5 mW at 1.3 μm was used. From experiments,an increase in responsivity by a factor 8 has been shown in thewaveguide configuration as compared to the normal incidence.

Applicants note that in this article a planar waveguide configuration ofthe photodiode is used, where the light is not confined laterally. Thelight signal travelling in the waveguide rapidly diverges in thewaveguide plane, where confinement is not present. Due to thisdivergence, the light intensity (Watt/cm²) decreases and it becomesnecessary to increase the area of the photodetector to maintain a goodefficiency. However, by increasing the photodetector area, the overallspeed of the device can be reduced.

In US patent application n. 2004/0188794 in the name of PrakashGothoskar et al., a photodetector for use with relatively thin (i.e.submicron) silicon optical waveguides formed in a silicon-on-insulatorstructure is disclosed. The photodetector comprises a layer ofpoly-germanium disposed to couple at least a portion of the opticalsignal propagating along the silicon optical waveguide. The siliconoptical waveguide may comprise any desired geometry, with thepoly-germanium detector formed to either cover a portion of thewaveguide or be butt-coupled to an end portion of the waveguide.

Applicants note that in all embodiments of the cited patent application,the electrical contacts are formed at the opposite ends of the detector,i.e. they are both in contact with the poly-germanium layer. Inaddition, the poly-Ge layer comprises a p-i-n structure, having ap-doped poly-germanium layer, an intrinsically doped layer and ann-doped layer. Applicants have observed that a p-i-n structure inpoly-Ge is difficult to realize, especially with deposition technologiesthat employ relatively low temperatures. Relatively low depositiontemperatures, i.e., not larger than 350-400° C., are desired in order topreserve compatibility with standard silicon CMOS technology.

SUMMARY OF THE INVENTION

The invention is relative to a waveguide photodetector structurecomprising a germanium layer, preferably a polycrystalline germaniumlayer (in the following shortened in “poly-Ge” layer), deposited on asilicon substrate. In particular, the selected photodetector geometry issuch that the performances of the device, with emphasis on its speed ofresponse and its responsivity, are optimised.

The photodetector of the invention comprises a heterojunction between alayer of poly-Ge and a silicon layer. Preferably, the heterojunction isa p-n junction between a p-type poly-Ge layer and an n-type siliconlayer. Near the p-n junction, electrons diffuse across to combine withholes, creating what is called a “depletion layer”. The surface in whichthe p-n junction is present (i.e. the area corresponding to thedepletion layer) is called “active area” of the photodetector structure.

Applicants have noted that one of the main drawbacks of poly-Gephotodetectors in normal incidence detection is the relatively smallthickness of the active layer of the poly-Ge heterojunction, e.g., about50 nm for thermally evaporated poly-Ge. This small thickness, regardlessof the poly-Ge layer total thickness, limits the responsivity of thedevice to a certain (small) value in normal incidence detection. Withthe term “normal incidence detection” it is meant that the light to bedetected is incident substantially perpendicularly to the plane definedby the heterojunction (i.e. the plane defined by the boundary betweenthe two different materials forming the junction).

In order to overcome the mentioned problem, a waveguide geometry isadopted in the present invention. In a “waveguide geometry”, opticalsignals to be detected travel in a waveguide, which may have anypreferred geometry, and are vertically confined therein. The dimensionsof the waveguide are such that an optical mode propagating in thewaveguide has an evanescent tail which extends beyond the waveguidelayer and thus the mode itself is sensitive to the presence ofadditional layer(s) eventually located on a surface of the waveguide.

The above-mentioned poly-Ge layer is deposited directly over thewaveguide, which is a silicon-based waveguide in order to form theaforementioned p-n junction: as the signal propagates along thewaveguide, it is coupled and then absorbed into the poly-Ge layer,creating electron-hole pairs. The presence of a buffer layer between thepoly-Ge layer and the Si waveguide, although less preferred than thedirect deposition of poly-Ge on Si, is not excluded. For instance, abuffer layer of Si slightly differently doped than the Si waveguidecould be included in the photodetector structure.

Thus, a photodetector structure having a “waveguide geometry” means thatoptical signals to be detected travel substantially along the samedirection of the interface between the poly-Ge and the waveguide, i.e.,the p-n junction, and optical absorption takes place along the lightpropagation path in the overlap region between the photodetector activearea and the guided mode profile of the mode travelling in thewaveguide. In other words, light travels substantially perpendicularlyto the direction of photogenerated carrier drift.

In this way, the light is absorbed into the thin sensitive layer of thepoly-Ge/Si heterojunction in a distributed way, during propagation. Thisreleases the strong constraint of device thickness being larger than1/α, (where α is the absorption coefficient) in order to maximizeabsorption efficiency.

The band gap of the germanium ensures effective absorption of nearinfrared (NIR) light. The photodetector structure of the invention istherefore preferably used for NIR light detection.

In addition, the use of a waveguide for light in-coupling is appealingfor telecom applications where signal is transported along an opticalfibre/waveguide. According to the invention, the photodetector structurehas a particular geometry of the poly-Ge layer, so that an opticalsignal is confined both vertically and laterally therein. In detail, thephotodetector comprises a rib structure, i.e., the poly-Ge layercomprises a mesa structure, called in the following simply “mesa” havinga given length L, a width W and a thickness T, where W<L. It is to benoted that within this context with mesa structure or mesa it is notnecessarily meant a planar top surface of poly-Ge (although preferred).For example, a rib-shaped or ridge-shaped poly-Ge layer could beenvisaged.

This mesa is in contact with the waveguide surface, preferably its topsurface, and the area W×L defines the active area of the photodetector,being the area in which light is absorbed. The mesa is located so thatthe length L is the length along the signal propagating direction in thewaveguide, while W is the width of the mesa in the directionsubstantially perpendicular to L. Light in the absorption region isconfined both vertically and laterally. In particular, light isvertically confined by the planar waveguide that has a higher refractionindex than that of the underlaying layer(s), while lateral guiding isobtained by the higher index of the lateral region corresponding to thepoly-Ge mesa (i.e., substantially corresponding to the width W of themesa). Two dimensional confinement preserves high light intensity(Watt/cm²) and allows smaller area devices with benefits in term ofspeed due to the reduced junction capacitance. In the preferredembodiments, the silicon waveguide forms with the underlaying layers asilicon-on-insulator (SOI) structure.

The electron-hole pairs generated due to the absorption of light can beefficiently collected using suitable metal contact structures fabricatedaccording to the teaching of the invention.

In the preferred embodiments, the thickness T of the poly-Ge layer, i.e.of the mesa, is chosen in such a way that a suitable compromise is foundbetween the losses due to the metal top contact(s), which suggest apreferred lower limit in thickness of poly-Ge layer, and the lossespresent in the Ge-region above one diffusion-length, which suggest apreferred upper limit in the thickness of the layer. In other words, alarge poly-Ge thickness increases the amount of light absorbed but notconverted into photocurrent at the contacts. However, due to thepresence of the metal contacts, T cannot be arbitrarily lowered due tothe increase of metal losses.

On one hand, the length L of the poly-Ge layer is preferably kept assmall as possible in order to achieve low capacitance and, therefore,preserve bandwidth. On the other hand, the length L is preferably longerthan the absorption length so that nearly complete absorption isobtained. The preferred range of the length of the poly-Ge mesa in thephotodetector of the present invention is 10 μm≦L≦2000 μm, morepreferably L is comprised between 400 μm≦L≦1000 μm.

The width W of the poly-Ge layer is preferably kept relatively low andin general minimized in order to limit the number of propagation modesand to maximize light intensity. However, calculation shows that lossesdue to the metallic contacts of the cathode (metal contact over thesilicon waveguide) cannot be neglected for W<10 μm. The width Wtherefore is preferably larger than 10 μm and it depends on the shape ofthe contacts themselves.

According to the invention, the photodetector comprises a first and asecond metal contact, the first metal contact (anode) is located abovethe poly-Ge mesa and the second metal contact (cathode) is located onthe silicon waveguide at both sides of the mesa.

Preferably, the first metal contact has a length equal to the length Lof the active area of the photodetector. More preferably, the firstmetallic contact is located on the top surface of the mesa symmetricallywith respect to the longitudinal axis of the mesa itself.

According to a first preferred embodiment, the first contact coverscompletely the poly-Ge mesa, i.e. it has a length L and a width W. Beingthe first metal contact relatively wide, metallic losses can berelevant. Preferably, according to the first embodiment, the thickness Tof the poly-Ge layer is comprised within 100 nm≦T≦160 nm in order toreduce the losses in the photodetector. Additionally, the preferredwidth W of the poly-Ge according to the first preferred embodiment is 10μm≦W≦50 μm.

The distance d between the first and the second metal contacts ispreferably minimized in order to minimize the series resistance of thephotodetector. However, the metal contacts should not be too laterallyclose one another to prevent parasitic capacitance. Preferably, 10μm≦d≦20 μm.

According to a second embodiment of the present invention, which differsfrom the first preferred embodiment only in the shape of the firstmetallic contact, this latter comprises two metallic strips located ontop of the poly-Ge mesa. Preferably, the first metal contact comprisestwo metal strips located symmetrically with respect to the poly-Ge layerand embracing the same. The two strips are connected with one another,such as in a U-shaped metal structure on top of the poly-Ge.

The first and second strips have a width w and are preferably disposedsymmetrically with respect to the longitudinal axis of the mesa. Morepreferably, the two metallic strips are at a relatively small distancefrom the edge of the top surface of the mesa.

The width w of the two metallic strips is preferably minimized in orderto minimize the metallic losses. Preferably, 2 μm≦w≦10 μm.

Due to the reduction of the metal losses in the second embodiment, theabove mentioned constraints in terms of the upper and lower limit for T′can be largely softened and the thickness T′ of the poly-Ge layer inthis second embodiment can be selected within a wider range of valuesthan in the first embodiment, preferably 60 nm≦T′≦200 nm and morepreferably 80 nm≦T′≦180 nm.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of a photodetector in germanium onsilicon according to the present invention will become more clear fromthe following detailed description thereof, given with reference to theaccompanying drawings, where:

FIG. 1 is a schematic prospective view of a first embodiment of thephotodetector of the present invention;

FIG. 2 is a schematic prospective view of a second embodiment of thephotodetector of the present invention;

FIG. 3 is a top view of the photodetector of FIG. 1;

FIG. 4 is a top view of the photodetector of FIG. 2;

FIGS. 5 a-5 c are lateral cross sectional views of the photodetector ofFIG. 1 (5 a), FIG. 2 (5 b) and of an additional embodiment of thephotodetector of the invention, showing the mode profile of the modetraveling in the waveguide and its overlap with the active layer;

FIG. 6 is a graph showing the calculated absorption efficiency (right)and the length (left) versus the polycrystalline germanium layerthickness for a W=10 μm of the poly-Ge layer in the photodetector ofFIG. 1. Contacts are in Au;

FIG. 7 is a graph showing the calculated absorption efficiency (right)and the length (left) versus the polycrystalline germanium layerthickness for a W=30 μm of the poly-Ge layer in the photodetector ofFIG. 2. Contacts are in Al and have a width w=5 μm;

FIG. 8 is a graphs of typical current-voltage characteristics of thephotodetector of FIG. 1 (solid line) and FIG. 2 (dashed line);

FIG. 9 is a graph showing typical wall-plug responsivity versus bias forboth horizontal (solid line) and vertical (dashed line) lightpolarization of the photodetector of FIG. 1;

FIG. 10 is a graph showing typical wall-plug responsivity versus biasfor both horizontal (solid line) and vertical (dashed line) lightpolarization of the photodetector of FIG. 2. The inset displays the bestresults in term of both responsivity and polarization independence;

FIG. 11 is a graph showing the photoresponse of device 1 to a modulatedlight input at 1.55 μm;

FIG. 12 is a graph of the metal losses of a waveguide for differentmetals used for the contacts;

FIGS. 13 a and 13 b are two schematic configurations of thephotodetector of the invention butt-coupled with an optical fiber, inthe embodiment of FIG. 5 b a piece of passive waveguide is alsocomprised;

FIG. 14 shows a schematic diagram of a silicon integrated circuitcomprising the photodetector, a transimpedance front-end and processingelectronics;

FIGS. 15 a-15 e show schematic diagrams of several applications of thephotodetector of the invention. In particular: FIG. 15 a is aconventional tap off configuration, 15 b is a wavelength selective ringresonator based sampling, FIG. 15 c is an in-line detection/monitoring,FIG. 15 d is a multiple tap off for wavelength division multiplexing(WDM) and FIG. 15 e is an improved selectivity version of WDM tap offbased on photonic crystals.

PREFERRED EMBODIMENTS OF THE INVENTION

With initial reference to FIG. 5 a, 1 indicates a photodetectorstructure realized according to the teaching of the present invention.

The photodetector structure 1 comprises a silicon-based waveguide 2 inwhich optical signals travels along a given direction X (which is alsothe axis of the waveguide) and are confined therein.

In particular, the optical signals are vertically confined in thewaveguide.

With the term “silicon-based” waveguide, it is meant a waveguiderealized in silicon, preferably n-type silicon material. Preferably, thewaveguide dimensions are such that the mode traveling therein is notfully confined vertically in the waveguide itself, but an evanescenttail of the mode extends outside the waveguide, so that the mode may beinfluenced by the location of additional layers on a waveguide'ssurface. Additionally, the waveguide geometry is arbitrary, i.e. a slab,rib or ridge waveguide, may be alternatively used depending on the finaldesired application of the photodetector structure 1. As an example, adifferent waveguide geometry is used in the photodetector depicted inFIG. 5 c.

Preferably, the waveguide 2 is realized on a layer 8, which has arefractive index lower than that of the waveguide. Preferably, layer 8is a SiO₂ layer. More preferably, the SiO₂ layer 8 is realized on asubstrate 3. Substrate 3, layer 8 and waveguide 2 form asilicon-on-insulator (SOI) structure, in which preferably the substrate3 is made of silicon and the waveguide (SOI overlayer) is n-doped Si.

Preferably, the thickness of the waveguide 2 is comprised between 2 and3 μm in order to obtain a convenient vertical confinement and minimallosses (i.e. in order to couple the fundamental vertical mode only andto make light coupling easier).

A polycrystalline Ge layer 4 is then grown on top of the silicon-basedwaveguide 2. Suitable techniques might be thermal evaporation,sputtering and chemical vapour deposition, being the first two preferredbecause of the inherent compatibility with silicon technology.Preferably, during poly-Ge deposition, the temperature of the substrateis kept above 250° C. (for example around 300-350° C.) so that thedeposited germanium layer has a polycrystalline structure. In this case,its absorption spectrum is very similar to pure (bulk) crystallinegermanium. The relatively low temperature at which the substrate is keptduring poly-Ge deposition allows a controlled deposition withoutaffecting either the functionality or the performance of possibleadditional Si electronics already present on the substrate.

Although the preferred embodiments of the present invention, which willbe described in details below, refer to polycrystalline germanium, aphotodetector device having a germanium layer with a (mono)crystallinestructure, i.e. c-Ge, or a crystalline layer of SiGe can be alsoincluded within the scope of the invention. Crystalline germanium orSiGe could be grown epitaxially on silicon. SiGe may have a Geconcentration ranging from a few percent to 100%, preferably not smallerthan 50%. However, crystalline Ge or SiGe are less preferred ifcompatibility with standard semiconductor technologies is desired, sincetheir fabrication generally require relatively high growth temperatures.

After deposition, the poly-Ge layer 4 is etched, in order to obtain thedesired geometry. According to the invention, the poly-Ge layer 4 isetched so as to form a mesa 10 having length L in the direction ofsignal propagation X in the waveguide and a width W in a direction Zsubstantially perpendicular to the propagation direction but laying inthe same plane. It is therefore identified a “detector area”, or “activearea” given by L×W in which light absorption takes place.

Additionally, the thickness of the poly-Ge mesa 10 is identified in thefollowing with T.

The combination of the silicon-based waveguide 2 in contact with thepoly-Ge layer 4 form a p-n heterojunction. Preferably, the poly-Ge layeris p-type and the Si-waveguide is n-type, however the reverse doping isalso included in the present invention (i.e., n-type poly-Ge on p-typeSi). A p-type germanium is preferred when thermal evaporation isemployed for fabrication of the layer 4, because the grown Ge layerturns out to be p-type without the need of an additional step of doping(e.g., by diffusion or ion implantation). Conversely, an n-type Ge layergrown by thermal evaporation and generally by other standard growthtechniques, such as chemical vapor deposition, would require an extraprocess step for the doping of the layer.

The absorption of the propagating light signal in the waveguide takesplace along the light propagating path where the poly-Ge layer, inparticular the mesa 10, is present. An active layer 6 (schematicallydepicted in FIGS. 5 a-5 c) is formed at the poly-Ge/Si interface: whenan incident photon reaches this region, due to the fact that theevanescent tail of the propagating mode in the waveguide 2 couples intothe poly-Ge layer 4, it “splits” in an electron-hole pair. The electronsthat are generated by the absorption of photons are collectedperpendicularly to the propagating mode vector by way for example of anelectric field applied at the p-n junction (reverse bias), as describedin the following.

Due to the fact that the photon absorption takes place along the signalpropagating path, the absorption efficiency mainly depends on thedetector length (i.e. on the poly-Ge mesa length L) rather than on thethickness of the active layer.

According to a first characteristic of the present invention, theoptical signal traveling in the waveguide 2, has to be confined bothlaterally and vertically in the poly-Ge layer 4. In particular, thepoly-Ge mesa dimensions are so selected that W<L. As already mentioned,two dimensional confinement preserves high light intensity (Watt/cm²)and allows smaller area devices to benefit from an improved speed due tothe reduced junction capacitance.

The optical lateral confinement may be due to the poly-Ge mesa geometry,however, the geometry of the silicon waveguide 2 can also influence theoptical lateral confinement. In a further embodiment of thephotodetector structure of the invention, which is illustrated in FIG. 5c and indicated as 1″, the waveguide 2 is etched in order to form aridge below the poly-Ge mesa 10 and thus enhancing the lateralconfinement.

Preferably, the length L of the poly-Ge mesa 10 is comprised between 10μm≦L≦2000 μm, more preferably between 400 μm≦L≦1000 μm. The preferred Lfor a specific application is selected according to the best compromisebetween the desired efficiency and speed of the device, which depend,among other, on W, L and the thickness T of the mesa.

The electrons generated by the light absorption are collected throughelectrodes, or contacts, appropriately located on the photodetector.

In a first preferred embodiment of the invention, shown in FIGS. 1, 3and 5 a, a first metal contact 7 is placed on a top surface 10 a of thepoly-Ge mesa 10. Preferably, the first contact 7 (anode) comprises ametal strip 15 disposed substantially parallel to the optical signalpropagation direction X (in the figures the X axis is depicted ascoincident to the longitudinal axis of the mesa 10). Even morepreferably, the metal strip 15 has a length equal to the length L of thepoly-Ge mesa 10. In particular, according to this first preferredembodiment, the first metal contact covers the poly-Ge layer 4completely, following the profile of its top surface 10 a and thusdefining a metallic strip of width W and length L.

A second metal contact (cathode) is placed on top of the silicon-basedwaveguide 2. The first and second contacts collect the electronsgenerated by the photon absorption in the active region L×W. Preferablythe second contact has the form of a strip. More preferably, the secondcontact comprises two metallic strips 9 a, 9 b located on the waveguide2 symmetrically with respect to the poly-Ge layer 4, i.e. they “embrace”the poly-Ge mesa 10 and lay parallel to the latter. The two symmetricmetallic strips 9 a, 9 b are also connected to each other.

According to a preferred embodiment of the invention, the photodetector1 comprises a metal pad 11 to more easily connect the first metalliccontact 7 to an external circuit (not shown) through wire bonding. Evenmore preferably, the poly-Ge layer 4 comprises a poly-Ge pad 12, fromwhich the mesa 10 projects, on top of which the metal pad 11 isdeposited, so that metal covers completely the upper surface of thepoly-Ge layer 4, as it can be clearly seen in FIG. 3. In this figure thepoly-Ge layer is not visible, being completely covered by the contact 7.Preferably, the poly-Ge pad 12 (and thus the metal pad 11) has a squareshape and for example its dimensions are at least 60×60 μm². In case ametal pad 11 is present, the active area of the photodetector 1 is givenby the L×W area plus the area of the pad.

The distance d between the first metallic contact 7 over the poly-Gemesa 10 and the second metallic contact 9 a or 9 b over thesilicon-based waveguide 2 is preferably minimized in order to minimizethe series resistance of the photodiode 1, thus increasing its opticalbandwidth. Applicants' calculations show that for a contacts' distancecomprised in the range between 10 and 20 μm, the series resistance isdominated by the contact resistance of the second metal contact 9 a, 9b. Thus, the preferred distance between the first and the second metalcontact is 10 μm≦d≦20 μm.

Applicants have noted that the thickness of the poly-Ge layer 4 is arelevant parameter when it comes to the photodetector responsivity,especially in the preferred embodiment of FIGS. 1, 5 a and 3. Themaximum thickness of the active layer in the poly-Ge layer is of about50 nm. Above this thickness, the absorbed light is lost (i.e. it doesnot contributed to detection). Indeed, electron-pairs that are generatedfar away from the depletion region travel primarily under the effect ofdiffusion and may recombine without giving rise to a current in theexternal circuit (connected to the photodetector through the contacts 7,9 a, 9 b). This reduces the efficiency of the photodetector 1.

Conversely, a too small value of the poly-Ge thickness results inincreased losses in the top metal contact 7. Since losses are due toabsorption in metal and in the poly-Ge layer above one-diffusion length,a trade-off is expected.

Simulations have been performed by Applicants on the device according tothe first embodiment of the invention which is illustrated in FIGS. 1, 3and 5 a, where the poly-Ge mesa 10 has W=10 μm and an L longer than theabsorption length. Results of simulations are shown in FIG. 6. The leftordinate represents the absorption length, while the right ordinate theabsorption efficiency. For each selected value of poly-Ge mesa thicknessT (which is the abscissa of the graph of FIG. 6, the thickness is givenin nm), the corresponding maximum absorption efficiency of thephotodetector 1 is calculated and visualized as a dot in the graph ofFIG. 6. For the same T-value, the absorption length (at 10 dB) iscalculated (the calculated absorption length can be considered as theminimum length necessary to achieve nearly complete absorption) and itis marked as a square in the aforementioned graph. It can be clearlyseen that the smaller the poly-Ge mesa thickness T, the longer is theabsorption length (in other words L is inversely proportional to T).

A too long absorption length is not desiderable because the devicebecomes bulky and thus less suitable for small devices realization, astypically required in integrated circuit technology. L is alsopreferably kept small in order to have low capacitance and, therefore,preserve bandwidth. The proposed preferred length range 400 μm≦L≦1000 μmhas been selected in order to be able to span up to total absorption(−10 dB) and up to a bandwidth of 10 GHz.

The expected trade-off caused by the above mentioned losses associatedto the presence of the metallic anode on top of the poly-Ge layer 4 canbe seen in the efficiency vs T curve of FIG. 6, which has a maximum atabout 120 nm.

More generally, after having selected a preferred value or range ofvalues of efficiency, which depends on the specific application of thephotodetector, a range of suitable thickness for the poly-Ge mesa isavailable. According to simulations, preferably the selected poly-Gethickness T is comprised between 100 nm and 160 nm.

Additionally, in this first embodiment of the invention, the preferredwidth W of the Poly-Ge mesa 10 is comprised between 10 μm≦W≦50 μm.Indeed, W is preferably minimized to limit the number of propagationmodes into the Poly-Ge layer 4 and keep light intensity high. Howeverlosses in the metallic contact 7 increase for W<10 μm, which can betherefore considered a lower limit.

Simulations suggest that the photodetector 1 of FIGS. 1, 3 and 5 a isrelatively sensitive to metal losses. Therefore, for the realization ofthe first and the second contact preferably a low loss metal is used.Preferred metals are Ag or Au. In FIG. 12, propagation losses due to thepresence of the metal contact 9 a,9 b on a silicon-based waveguide 2 ona SiO₂ substrate 3 are shown for different contact metals.

According to different embodiments of the present invention, not shown,the first metal contact 7 may have the form of a metal strip of lengthL, but its width may be smaller than the width W of the poly-Ge mesa 10.In particular the contact 7 may have the form of a strip disposed on topof the poly-Ge mesa 10 parallel to the signal propagating direction Xand extending along the length L, and it may be located in proximity ofa border of the mesa upper surface, i.e. it may be in contact to theborder of the rectangle W×L or it can be located at a given distancefrom that border. However, the presence of a single central strip wouldperturb the propagating optical mode around its maximum, driving theoptical field away from the metal strip and thereby reducing efficiency.

The value of the dark current of the photodiode 1 has importantimplications on the overall power dissipation and noise performancethrough the associated shot noise. Typical measured dark currentdensity/reverse bias (applied at the heterojunction) characteristics ofthe photodetector 1 of FIGS. 1, 3 and 5 a are displayed in FIG. 8. Thesolid line represents the dark current density in the first preferredembodiment under issue. Therefore, the photodetector of the presentembodiment exhibits a good reverse-bias behavior and dark currentdensities I_(d) of 0.6 and 1 mA/cm² at 1 and 10 V, respectively, with astandard deviation <10%. The corresponding shot noise is evaluated fromthe measured I_(d) according to i_(s)=(2qI_(d)B)^(1/2), where q is theelectron charge and B the bandwidth (considered in this calculationequal to 1 GHz). Typical values for a photodetector according to thefirst embodiment having W=9 μm, L=1 mm, T=120 nm (bonding pad is 10⁻³cm²) are listed in Table 1: TABLE 1 Reverse Bias (V) I_(d)(A) I_(s)(nA)1 4.2*10⁻⁷ 11 10 6.6*10⁻⁷ 14 20 1.7*10⁻⁶ 23

The responsivity of the photodetector of this embodiment has beenmeasured by butt-coupling light from a semiconductor laser through a 40×objective and a half-waveplate for polarization. The photocurrent wasconverted to voltage and then measured by a lock-in amplifier versusreverse bias. Measurements of wall-plug responsivities defined as theratio between the photocurrent and power at the lens input (i.e. with nocorrection for lens transmission, reflection and modal mismatch) areperformed. Typical responsivities of photodetectors with T=120 nm, W=8μm, L=600 μm at 1.55 μm versus reverse bias are shown in FIG. 9, forboth horizontal (solid line) and vertical (dashed line) lightpolarization. It can be seen that wall-plug responsivities is comprisedin the 10-15 mA/W range.

Finally, speed measurements have been performed. If the device isappropriately biased, the transit time associated to the crossing of thedepletion layer is expected in less than hundred picoseconds and thebandwidth of the photodetector 1 is thought to be RC-limited, where R isthe series resistance (R_(S)) plus load resistance R_(L) and C thejunction capacitance. Above 10 V reverse-bias, the silicon waveguide(having a thickness in the aforesaid measurements of 2 μm) is assumed tobe totally depleted and the inferred junction capacitance is between 0.8and 2.0 pF for device having L between 400 μm and 1200 μm and a 60×60μm² metal pad. According to this range of capacitances and taking intoaccount a 100 Ohm of load and series resistance, a bandwidth between 0.5GHz and 2 GHz is expected. FIG. 11 displays the typical measuredresponse for the photodetector 1 (with W=10 μm, L=100 μm and T=120 nm,bonding pad is 60×60 μm²) of FIGS. 1, 3 and 5 a, and the plotted curveshows rise and fall times of about 500 ps. Rise time of 500 pscorresponds to bandwidth of approximately 0.7 GHz, which is in theexpected range. Nevertheless, the measured series resistance R_(S)exceeded 500 Ohm and it could be minimized (thus increasing the devicespeed) optimizing the ohmic character of the cathode contact.

Example 1

A Bond and Etch-back SOI substrate with 2 μm-thick n-type Si overlayer(2-3 Ωcm) and 1.5 μm-thick SiO₂ insulator has been used. The sampleswere cleaved, but the input facets were un-polished.

Polycrystalline Ge film was grown by thermal evaporation using a99.999%-purity commercial source and a tungsten crucible in vacuum witha background pressure of 10⁻⁶-10⁻⁷ Torr. The substrate was heated by amassive copper-plate, temperature-stabilized at 300° C. by athermo-coaxial cable. The Ge film thickness T was selected equal to 120nm. Before the introduction into the vacuum chamber, the SOI substratewas chemically cleaned at room temperature by dipping them into bufferedhydrofluoric acid (BHF) for 10 s. Poly-Ge mesa 10 was defined bylithography and selective wet-etching.

Aluminum contacts were subsequently deposited by thermal evaporation anddefined by standard lithography. The square pad for wire bonding is ofabout 60×60 μm².

Several devices has been fabricated having different lengths between 400and 2400 μm.

A second preferred embodiment of the photodetector according to theinvention, indicated as 1′ in the figures, is shown in FIGS. 2, 4 and 5b. An additional variant of this embodiment is shown in FIG. 5 c wherethe geometry of the waveguide 2 is different from the geometry depictedin FIG. 5 b. However this ridge waveguide of FIG. 5 c may bealternatively employed also in the framework of the photodetector of thefirst embodiment. Identical reference numerals indicate in FIGS. 2, 4and 5 b/5 c functionally identical parts with respect to the firstpreferred embodiment indicated with 1 of FIGS. 1, 3 and 5 a.

The difference between the photodetector 1 and the photodetector 1′ liesin the different shape and dimensions of the first metal contactdeposited over the poly-Ge layer 4. In the photodetector 1′ according toa second preferred embodiment, the first metal contact on the poly-Gemesa comprises two metallic strips 16 a, 16 b, both having preferablylength L and being disposed parallel one with respect to the other andalso parallel to the propagating direction X. Preferably, the two strips16 a, 16 b are disposed symmetrically with respect to the longitudinalaxis X on top of the mesa 10. The width of these strips, indicated withw, is smaller than W/2. More preferably, the two strips have identicallateral dimensions.

This second embodiment of a photodetector structure 1′ having two strips16 a, 16 b located symmetrically with respect to the centre of the mesaupper surface 10 a is preferred. It is to be noted that metal losses aremuch more relevant in the first embodiment (i.e., photodetector 1) thanthose in the second embodiment due to the larger contact dimensions ofthe former. A single lateral stripe could be used, but the resultingelectric field distribution would not be optimised for photocarriercollection and series resistance might increase.

The two strips 16 a, 16 b are connected to each other, for example bymeans of the metal pad 11 (FIG. 4).

The second metallic contact 9 a, 9 b on the silicon-based waveguide 2remains unchanged with respect to the first preferred embodiment.

Preferably, the two strips 16 a, 16 b are not adjacent to the boundaryof the mesa 10, but there is a minimal distance between the edge of eachstrip and the edge of the mesa 10, to prevent the risk of short-cuts dueto metal portions belonging to the first metal contact 7 contactingaccidentally the silicon waveguide 2. This minimal distance ispreferably of the order of 1 μm.

In the first embodiment of the photodetector of the invention, it hasbeen shown (see FIG. 6) that losses due to the metal contacts arerelatively relevant and for this reason the thickness T of the poly-Gelayer 4 is preferably not too small (i.e. it is thicker than about 100nm). In this second preferred embodiment, the amount of metal present onthe poly-Ge layer 4 is reduced with respect to the first embodiment andthus it is expected that losses in contacts are negligible, or at leastreduced.

As depicted in FIG. 7, the analogue curve for the second embodiment 1′of the curve depicted in FIG. 6 for the first preferred embodiment 1 isshown. In particular, simulations of the behaviour of a photodetector 1′having a poly-Ge mesa 10 of W=30 μm and a width of the two metal strips16 a, 16 b both equal to w=5 μm are shown. The contacts 7, 9 a, 9 b arerealized in aluminium. Due to the decrease of the metal losses, theefficiency of the photodetector 1′ shows a monotonic behaviour versuspoly-Ge layer thickness T′ (in the first embodiment, this curve has amaximum). This latter parameter T′ is therefore only limited byfabrication constraints as the roughness of the layer and by the minimumvalue equal to the active layer close to the Ge/Si interface (which isof about 50 nm) in order to have nearly complete absorption.

The preferred thickness T′ of the poly-Ge layer 4 in this secondpreferred embodiment is therefore comprised between 60 nm≦T′≦200 nm andmore preferably between 80 nm≦T′≦180 nm. This relatively wide range ofpreferred suitable thickness of poly-Ge layer allows more designflexibility and an expected improvement in responsivity. Again, atrade-off between responsivity and speed is generally selected.

On the other hand, due to the more complex geometry and to technologicalconstraints given by the realization of two tiny parallel metal stripson the same surface of the Poly-Ge mesa 10, preferably the poly-Ge mesa10 has a width W′ wider than or equal to 20 μm. More preferably, thewidth of the mesa in this embodiment is 20 μm≦W′≦50 μm.

The width w of the strips 16 a, 16 b is preferably minimized to have alarge covered-uncovered ratio, and it is also limited by thelithographic resolution. Preferably, 2 μm≦w≦10 μm.

The other preferred design parameters of the photodetector 1′ of thissecond embodiment are equal to the ones mentioned with regard to thefirst embodiment 1, such as the value of the length L and the shape ofthe second metal contact 9 a,9 b.

The values listed in of Table 1 calculated for this second embodiment,in particular for a photodetector 1′ having W=30 μm, w=5 μm, L=500 μmand T=120 nm, are shown below in Table 2: TABLE 2 Reverse Bias (V)I_(d)(A) I_(s)(nA) 1 5.2*10⁻⁷ 13 10 6.3*10⁻⁶ 44 20 8.6*10⁻⁵ 165

The typical measured current-voltage characteristics for the secondembodiment of photodetector 1′ is shown in FIG. 8 where the curve forthe second embodiment is plotted using a dashed line.

The typical measured wall-plug responsivity versus bias voltage for bothhorizontal (solid line) and vertical (dashed line) light polarizationsat 1.55 μm for the photodetector 1′ is shown in FIG. 10. The insetdisplays the best results in term of both responsivity and polarizationindependence. Wall-plug responsivity are in the 8-12 mA/W range fordetectors with W=30 μm, w=5 μm, L>500 μm and T=120 nm.

It is also to be noted that the polarization responsivity ratio istypically reduced from 5.5 (photodetector 1 of the first embodiment, seeFIG. 9) down to 1.6 (photodetector 1′ of the second embodiment, see FIG.10). This is associated with the absence of a top-metal on most of thepoly-Ge layer 4 with double contacts substantially U-shaped.

Due to the high conductivity of the poly-Ge layer 4, the junctioncapacitance of the photodetector 1′ of this second embodiment issubstantially equal to the junction capacitance of the photodetector ofthe first embodiment, even if the contact area is different.Accordingly, the two devices exhibit an almost identical speed (see FIG.11 for the speed of the photodetector 1 of the first embodiment). Thephotodetectors 1, 1′, 1″ of the invention may be used as discretecomponents of optical fiber communication receivers. The device 1, 1′,1″ can be directly coupled with an optical fiber 51 (FIG. 13 a) orcoupled through a piece of waveguide 50 which allows routing and bendingthe incoming light (see FIG. 13 b). They can be also used as part ofvarious higher-level systems. The compatibility with standard VLSIsilicon technology allows the fabrication of silicon basedoptoelectronic integrated circuits as the receiver 100 shown in FIG. 14.The receiver 100 comprises the photodetector 1 (or 1′ or 1″), an analogfront-end (transimpedance amplifier) 52 and some processing/addressingmixed-signal electronics 53.

Different systems can be employed using the photodetector of theinvention to tap off a particular wavelength from an incoming opticalsignal for either monitoring purpose or for complete drop off. Thislight measurement can be both wideband and narrowband (wavelengthselective).

FIGS. 15 a-15 e show different configurations which can utilize thephotodetector of the invention. FIG. 15 a shows a conventionalconfiguration which can be designed for both wideband and narrowband tapoff, FIG. 15 b shows a wavelength selective ring resonator basedsampling, FIG. 5 c shows an in-line configuration which allows lightmonitoring which leaves the signal substantially the same, FIG. 15 dshows a multiple tap off/detection for wavelength division multiplexing(WDM), and

FIG. 15 e shows an improved version of WDM receiver/monitor based onphotonic crystals which greatly improve wavelength selectivity. In thementioned figures, the photodetector is indicated with 1 for sake ofconciseness, but either a photodetector according to the firstembodiment 1 of the invention, or according to the second 1′, 1″embodiment can be alternatively selected.

1-38. (canceled)
 39. A photodetector structure, comprising: asilicon-based waveguide in which optical signals to be detected travelin a given direction and are confined therein; a germanium layerdisposed on a portion of said silicon-based waveguide so that anevanescent tail of the propagating optical signal in said waveguide iscoupled into said germanium layer, said germanium layer comprising amesa having a length along the signal propagating direction and a widthin a direction substantially perpendicular to the propagating direction,and in which the width of said mesa is smaller than its length; and afirst and a second metal contact, said first metal contact being locatedon said germanium layer and said second metal contact being located onsaid silicon-based waveguide, said first and second contacts being usedto collect electrons generated by light absorption to obtain an outputelectric signal.
 40. The photodetector structure according to claim 39,wherein said germanium layer is a polycrystalline germanium layer. 41.The photodetector structure according to claim 39, wherein saidgermanium layer is disposed directly in contact with a portion of saidsilicon-based waveguide.
 42. The photodetector structure according toclaim 39, wherein said first metal contact comprises a metallic striplocated on top of said silicon-based waveguide.
 43. The photodetectorstructure according to claim 42, wherein said metallic strip of saidfirst contact is disposed parallel to said propagating direction and hasa length along said propagating direction substantially equal to thelength of said germanium mesa.
 44. The photodetector structure accordingto claim 40, wherein the length of said germanium mesa is between 10μm≦L≦2000 μm.
 45. The photodetector structure according to claim 44,wherein the length of said germanium mesa is between 400 μm≦L≦1000 μm.46. The photodetector structure according to claim 39, wherein saidsecond metal contact comprises a metallic strip located on top of saidsilicon-based waveguide.
 47. The photodetector structure according toclaim 39, wherein said second metal contact comprises two metal stripslocated on top of said silicon-based waveguide and disposedsymmetrically with respect to said germanium layer.
 48. Thephotodetector structure according to claim 39, comprising a layer andwherein said silicon-based waveguide is located on top of said layerwhich is realized on a substrate, said waveguide, said layer and saidsubstrate forming a silicon-on-insulator structure.
 49. Thephotodetector structure according to claim 48, wherein said layer is aSiO₂ layer.
 50. The photodetector structure according to claim 48,wherein said substrate comprises silicon.
 51. The photodetectorstructure according to claim 39, wherein said silicon-based waveguide isn-type.
 52. The photodetector structure according to claim 39, whereinsaid germanium layer is p-type.
 53. The photodetector structureaccording to claim 39, wherein the thickness of said silicon-basedwaveguide is 2 to 3 μm.
 54. The photodetector structure according toclaim 40, wherein the width of said germanium mesa is between 10 μm≦W≦50μm.
 55. The photodetector structure according to claim 42, wherein saidmetallic strip of said first contact has a width substantially equal tothe width of said polycrystalline germanium layer.
 56. The photodetectorstructure according to claim 40, wherein the thickness of said germaniummesa is between 100 nm≦T≦160 nm.
 57. The photodetector structureaccording to claim 42, wherein said metallic strip of said first contacthas a width smaller than the width of said germanium layer.
 58. Thephotodetector structure according to claim 40, wherein said firstmetallic contact comprises a first and a second metallic strip disposedon said germanium layer.
 59. The photodetector structure according toclaim 58, wherein said first and second metallic strips are positionedone parallel to the other.
 60. The photodetector structure according toclaim 58, wherein said first and second metallic strips both havesubstantially the same length as said germanium mesa.
 61. Thephotodetector structure according to claim 58, wherein said first andsecond strips have a width between 2 μm≦w≦10 μm.
 62. The photodetectorstructure according to claim 61, wherein said first and second stripshave equal width.
 63. The photodetector structure according to claim 58,wherein the width of said germanium mesa is between 20 μm≦W′≦50 μm. 64.The photodetector structure according to claim 58, wherein saidgermanium mesa comprises a top surface and said first and second stripsare disposed symmetrically on said top surface.
 65. The photodetectorstructure according to claim 58, wherein said germanium mesa comprises atop surface and said first and second strips are located at a givendistance from the lateral edge of said top surface.
 66. Thephotodetector structure according to claim 65, wherein said givendistance is of the order of 1 μm.
 67. The photodetector structureaccording to claim 58, wherein the thickness of said germanium mesa isbetween 60 nm≦T′≦200 nm.
 68. The photodetector structure according toclaim 67, wherein the thickness of said germanium mesa is between 80nm≦T′≦180 nm.
 69. The photodetector structure according to claim 39,wherein the distance between said first and said second metal contactsis between 10 μm≦d≦20 μm.
 70. The photodetector structure according toclaim 39, wherein said germanium layer comprises a pad from which saidmesa extends.
 71. The photodetector structure according to claim 70,wherein a metal pad is located above said pad realized in said germaniumlayer.
 72. The photodetector structure according to claim 71, whereinsaid metal pad is square.
 73. The photodetector structure according toclaim 39, wherein said first and said second metal contacts comprisegold.
 74. The photodetector structure according to claim 39, whereinsaid first and said second metal contacts comprise silver.
 75. Anoptical fiber communication receiver comprising the photodetectorstructure according to claim
 39. 76. An optical filter comprising thephotodetector structure according to claim 39.